Rubber composition comprised of cis-1,4-polyisoprene and polymeric di-maleamic acid and articles, including tires, having at least one component comprised thereof

ABSTRACT

This invention relates to rubber compositions comprised of cis-1,4-polyisoprene and a polymeric di-maleamic acid and to articles of manufacture having at least one component comprised thereof, including tires. The polymeric di-maleamic acid has been observed to enhance green strength of such rubber composition.

FIELD

This invention relates to rubber compositions comprised ofcis-1,4-polyisoprene and a polymeric di-maleamic acid and to articles ofmanufacture having at least one component comprised thereof, includingtires. The polymeric di-maleamic acid has been observed to enhance greenstrength of such rubber composition.

BACKGROUND OF THE INVENTION

Unvulcanized cis-1,4-polyisoprene elastomers, both natural andsynthetic, particularly the synthetic elastomer, typically do not havesufficiently desirable green strength in order to permit assembling, orbuilding, various rubber composition-based components which contain suchelastomers to form articles of manufacture such, as for example, tires.

A typical major deficiency of unvulcanized syntheticcis-1,4-polyisoprene, and therefore various rubber compositions whichcontain unvulcanized synthetic cis-1,4-polyisoprene is a usual lack ofsufficient green strength and tack needed for satisfactory processing orbuilding properties required in the building of articles of manufactureincluding the building of tires. The abatement of such usual deficiencyhas often been sought and may assist in facilitating a replacement, orat least a partial replacement, of natural rubber for appropriate rubbercompositions.

The term “green strength”, while being commonly employed and generallyunderstood by persons skilled in the rubber industry, is nevertheless adifficult property to precisely define. Basically, green strength may bethought of as the tensile strength developed when an unvulcanizedpolymer composition of proper configuration is stressed under controlledconditions. Beyond an initial yield point, unvulcanized naturalcis-1,4-polyisoprene rubber compositions will show increasing strengthagainst rupture, or significant deformation, while unvulcanizedsynthetic cis-1,4-polyisoprene will typically fall below the yield pointor will increase only slightly above it. In certain practicalapplications such as uncured tires, belting, shoes and a number of otherproducts in the course of manufacture, green strength is important inpromoting the integrity and cohesiveness, including dimensionalstability, of the assembly of various rubber components between buildingor assembly thereof and the ultimate molding and accompanyingvulcanization of the assembled article.

Green strength often manifests itself secondarily in the tack oradhesiveness imparted to various unvulcanized rubber compositionsemployed in the manufacture of a number of rubber articles such astires, belting, etc. Other things being equal, an unvulcanized rubber orrubber composition having higher green strength will often exhibitbetter building tack or adhesion to other unvulcanized rubber-basedcomponents and will accordingly ease various fabrication, processing andhandling problems associated with the building and the ultimate moldingand vulcanization of fabricated articles.

Various additive compounds or agents which have heretofore been utilizedto improve green strength of synthetic rubber elastomers, for example,numerous nitroso compounds as mentioned in U.S. Pat. Nos. 2,457,331,2,477,015, 2,518,576, 2,526,504, 2,540,596, 2,690,780 and 3,093,614.Additionally, various compounds have been mentioned such as thosedescribed in U.S. Pat. Nos. 2,969,341, 3,037,954, 3,160,595 and BritishPatent No. 896,309. Yet another class of additives or compounds are thediesters of 5-norbonene as mentioned in U.S. Pat. Nos. 3,817,883 and3,843,613. In U.S. Pat. No. 4,124,750 a dihydrazide compound issuggested for cross-linking a synthetic rubber to enhance its greenstrength.

In the description of this invention, the term “phr” is used todesignate parts by weight of a material per 100 parts by weight ofelastomer. In the further description, the terms “rubber” and“elastomer” may be used interchangeably unless otherwise mentioned. Theterms “vulcanized” and “cured” may be used interchangeably, as well as“unvulcanized” or “uncured”, unless otherwise indicated.

SUMMARY AND PRACTICE OF THE INVENTION

In accordance with this invention, a rubber composition comprised ofcis-1,4-polyisoprene is provided which contains from about 0.1 to about20, alternately about one to about 10, phr of a polymeric di-maleamicacid.

Preferably said cis-1,4-polyisoprene elastomer is syntheticcis-1,4-polyisoprene because it typically possesses less green strengththan natural cis-1,4-polyisoprene elastomer.

Alternately, the cis-1,4-polyisoprene may constitute both the naturaland synthetic cis-1,4-polyisoprene elastomers or may simply be a naturalrubber.

It is to be appreciated that such rubber composition may contain otherconjugated diene-based elastomers such as, for example homopolymers andcopolymers of isoprene (other than cis-1,4-polyisoprene) and1,3-butadiene and copolymers of isoprene and 1,3-butadiene with a vinylaromatic compound such as styrene and/or alphamethyl styrene, preferablystyrene.

In further accordance with this invention, a rubber compositioncomprises, based upon 100 parts by weight elastomers (phr),

(A) 100 phr of unvulcanized conjugated diene based elastomers comprisedof

(1) about 10 to about 90, alternately about 20 to about 80, phr ofcis-1,4-polyisoprene elastomer and, correspondingly

(2) about 90 to about 10, alternately about 80 to about 20, phr of atleast one additional conjugated diene-based elastomer, and

(B) about 0.1 to about 20, alternately about one to about 10, phr ofpolymeric di-maleamic acid.

While the mechanism may not be entirely understood, in practice it isbelieved that the polymeric di-maleamic acid may, to a degree, createsome crosslinks between the olefinic portion of the maleamic acid andthe olefinic portion of the unvulcanized cis-1,4-polyisoprenediene-based elastomer to thereby increase the green strength of theoverall rubber composition.

The resulting rubber compositions may be used as various components ofarticles of manufacture, particularly various components of tires suchas, for example sidewall, carcass ply, tread and apex.

The polymeric di-maleamic acid may be characterized as apolyoxypropylene polymer of about 200 to about 10,000 molecular weightwith unsaturated half amide-acid functional groups on each end.

Such polymeric di-maleamic acid might be prepared, for example, byreacting a 2000 molecular weight polyoxypropylene polymer (e.g.Jeffamine D2000® from the Texaco Company) which contains terminalprimary amino groups (one on each end of the polymer) with two molarequivalents of maleic anhydride in acetone solution with reflux for 14hours before the solvent is removed under reduced pressure. Theresulting polymeric di-maleamic acid is a polyoxypropylene backbonepolymer with a maleamic acid termination on each end of the polymer. Theterminal di-maleamic acid functional groups arise from the reaction ofthe two molar equivalents of maleic anhydride with the two terminalprimary amino groups of the polyoxypropylene.

In the practice of this invention, as hereinbefore pointed out, therubber composition of this invention may contain at least one additionaldiene-based elastomer. Thus, it is considered that the elastomer is asulfur curable elastomer.

The additional diene based elastomer may be selected, for example, fromhomopolymers and copolymers of at least one diene selected from isopreneand 1,3-butadiene (other than the aforesaid natural and syntheticcis-1,4-polyisoprene) and copolymers of at least one diene selected fromisoprene and 1,3-butadiene with a vinyl aromatic compound selected fromat least one of styrene and alphamethyl styrene, preferably styrene.

Representative of such additional elastomers are, for example, at leastone of styrene/butadiene copolymer elastomers (aqueous emulsionpolymerization derived and organic solvent solution polymerizationderived elastomers), isoprene/butadiene copolymer rubbers,styrene/isoprene copolymer rubbers, styrene/isoprene/butadieneterpolymer rubbers, cis-1,4-polybutadiene rubber, high vinylpolybutadiene rubber with a vinyl 1,2- content in a range of about 30 toabout 90 percent, emulsion polymerization preparedbutadiene/acrylonitrile copolymers and a minor amount of3,4-polyisoprene rubber.

The rubber composition may be preferably comprised of at least twoadditional conjugated diene-based based elastomers.

In the further practice of this invention, particulate reinforcement forthe rubber composition may be rubber reinforcing carbon black, amorphoussilica, or a combination of such carbon black and silica, usually of anamount in a range of about 35 to about 100 alternately about 35 to about90, phr. If a combination of such carbon black and silica is used,usually at least about 5 phr of carbon black and at least 10 phr ofsilica is used. For example, a weight ratio of silica to carbon blackranging from about 5/1 to 5/1 might be used.

Commonly employed amorphous silica, or siliceous pigments, used inrubber compounding applications can be used as the silica in thisinvention, including pyrogenic and precipitated siliceous pigments areprecipitated and fumed silica wherein precipitated silicas are usuallypreferred.

The siliceous pigments preferably employed in this invention areprecipitated silicas such as, for example, those obtained by theacidification of a soluble silicate, e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,Page 304 (1930).

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 50 to about400 cm3/100 g, and more usually about 100 to about 300 cm3/100 g.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available from Rhodiaas, for example, Zeosil 1165MP and Zeosil 165GR, silicas available fromDegussa AG with, for example, designations VN2 and VN3, as well as othergrades of silica, particularly precipitated silicas, which can be usedfor elastomer reinforcement.

It is readily understood by those having skill in the art that therubber composition would be compounded by methods generally known in therubber compounding art, such as mixing the various sulfur-vulcanizableconstituent rubbers with various commonly used additive materials suchas, for example, curing aids, such as sulfur, activators, retarders andaccelerators, processing additives, such as oils, resins includingtackifying resins, silicas, and plasticizers, fillers, pigments, fattyacid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agentsand reinforcing materials such as, for example, carbon black. As knownto those skilled in the art, depending on the intended use of the sulfurvulcanizable and sulfur vulcanized material (rubbers), the additivesmentioned above are selected and commonly used in conventional amounts.

Typical amounts of tackifier resins, if used, comprise about 0.5 toabout 10 phr, usually about 1 to about 5 phr. Typical amounts ofprocessing aids comprise about 1 to about 50 phr. Such processing aidscan include, for example, aromatic, napthenic, and/or paraffinicprocessing oils. Typical amounts of antioxidants comprise about 1 toabout 5 phr. Representative antioxidants may be, for example,diphenyl-p-phenylenediamine and others, such as, for example, thosedisclosed in The Vanderbilt Rubber Handbook (1978), Pages 344 through346. Typical amounts of antiozonants comprise about 1 to 5 phr. Typicalamounts of fatty acids, if used, which can include stearic acid compriseabout 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 1to about 10 phr. Typical amounts of waxes comprise about 1 to about 5phr. Often microcrystalline waxes are used. Typical amounts of peptizerscomprise about 0.1 to about 1 phr.

The vulcanization is conducted in the presence of a sulfur vulcanizingagent. Examples of suitable sulfur vulcanizing agents include elementalsulfur (free sulfur) or sulfur donating vulcanizing agents, for example,an amine disulfide, polymeric polysulfide or sulfur olefin adducts.Preferably, the sulfur vulcanizing agent is elemental sulfur. As knownto those skilled in the art, sulfur vulcanizing agents are used in anamount ranging from about 0.5 to about 4 phr, or even, in somecircumstances, up to about 8 phr.

Accelerators are used to control the time and/or temperature requiredfor vulcanization and to improve the properties of the vulcanizate. Inone embodiment, a single accelerator system may be used, i.e., primaryaccelerator. Conventionally and preferably, a primary accelerator(s) isused in total amounts ranging from about 0.5 to about 4, preferablyabout 0.8 to about 1.5, phr. In another embodiment, combinations of aprimary and a secondary accelerator might be used with the secondaryaccelerator being used in smaller amounts (of about 0.05 to about 3 phr)in order to activate and to improve the properties of the vulcanizate.Combinations of these accelerators might be expected to produce asynergistic effect on the final properties and are somewhat better thanthose produced by use of either accelerator alone. In addition, delayedaction accelerators may be used which are not affected by normalprocessing temperatures but produce a satisfactory cure at ordinaryvulcanization temperatures. Vulcanization retarders might also be used.Suitable types of accelerators that may be used in the present inventionare amines, disulfides, guanidines, thioureas, thiazoles, thiurams,sulfenamides, dithiocarbamates and xanthates. Preferably, the primaryaccelerator is a sulfenamide. If a second accelerator is used, thesecondary accelerator is preferably a guanidine, dithiocarbamate orthiuram compound.

The presence and relative amounts of the above additives are notconsidered to be an aspect of the present invention, unless otherwiseindicated herein, which is more primarily directed to the utilization ofpolymeric di-maleamic acid in various rubber compositions which containcis-1,4-polyisoprene, particularly synthetic cis-1,4-polyisopreneelastomer.

The following examples are presented to illustrate the invention and arenot intended to be limiting. The parts and percentages are by weightunless otherwise designated.

EXAMPLE 1

A rubber composition comprised of an unvulcanized syntheticcis-,4-polyisoprene elastomer and polymeric di-maleamic acid wasprepared by mixing in an internal rubber mixer for about three minutesto a temperature of about 160° C. as shown in the following Table 1 asrepresented herein as Sample B.

Sample A is presented as a Control Sample where the unvulcanizedsynthetic cis-1,4-polyisoprene elastomer is blended in an internalrubber mixer with a conventional rubber processing oil.

TABLE 1 Parts Sample A Material Control Sample B Non-Productive MixingStep No. 1 (3 minutes to 160° C.) Cis-1,4-polyisoprene¹ 100 100 Carbonblack² 50 50 Processing oil³ 5 0 Polymeric di-maleamic acid⁴ 0 5 Zincoxide 5 5 Stearic acid 2 2 Non-Productive Mixing Step No. 2 (3 minutesto about 160° C.) No added ingredients in this mixing step ProductiveMixing Step (2 minutes to about 108° C.) Sulfenamide Accelerator 1 1Sulfur 1.4 1.4 ¹Synhetic cis-1,4-polyisoprene obtained as NAT 2200 fromThe Goodyear Tire & Rubber Company ²N299 carbon black, an ASTMdesignation ³Naphthenic/paraffinic rubber processing oil obtained asFlexon 641 from the Exxon Mobil Company ⁴Polymeric di-maleamic acid wasprepared by reacting a polyoxypropylene diamine with two molarequivalents of maleic acid as hereinbefore described. GPC (GelPermeation chromatography) analysis showed that the polymer has 6.0percent of its molecular weight in the 10,200 range, 19.3 percent in the6,710 range, 73.4 percent in the 3,150 range, 0.4 percent in the 430range and 0.8 percent in the 210 range.

Various physical properties of the treated syntheticcis-1,4-polyisoprene rubber composition of Table 1 were evaluated andreported in the following Table 2.

TABLE 2 Parts Sample A Properties Control Sample B UnvulcanizedProperties Tensile strength (MPa) 0.1 0.22 Elongation (%) 1477 413 GreenStrength (MPa) 40% elongation 0.21 0.37 80% elongation 0.24 0.5 120%elongation 0.21 0.49 Vulcanized Properties (36 minutes at 150° C.)Rheometer T90 13.8 12 Torque min (dNm) 7.5 9.1 Torque max (dNm) 38.239.1 Delta Torque (dNm) 39.7 30 Modulus, MPa 100% 2.06 2.2 300% 10.8 11Ult Tensile strength (MPa) 23.4 23.4 Ult Elongation (%) 567 568 Hardness(Shore A) 23° C. 61.9 45.5 100° C. 56.6 57.8 Rebound, % 23° C. 46.7 45.5100° C. 63.5 60.9 Strebler Adhesion 95° C. 160 159 DIN Abrasion¹ 136 154E' at 60° C. (MPa) 16.1 17.3 Tan Delta 0.095 0.095 ¹Relative volume losswhereas a lower value represents a lower volume loss and therefore abetter resistance to abrasion.

It can readily be seen from Table 2 that the green strength of Sample Bis significantly greater than that of Control Sample A. The greenstrengths demonstrated herein are the force(s) used for the variouselongations at about 23° C.

This is considered herein to be significant because an improved greenstrength for an unvulcanized rubber composition which contains anappreciable content of synthetic cis-1,4-polyisoprene rubber is desiredfor preparation, or building, of many articles of manufacture, includingtires.

It also can readily be seen from Table 2 that the rebound and DINabrasion of the vulcanized rubber for Sample B are substantiallymaintained, as compared to Control Sample A, even though the greenstrength of the unvulcanized synthetic cis-1,4-polyisoprene rubber issubstantially, and beneficially, increased.

This is considered herein to be significant because it is often desiredto increase the green strength of various rubber compositions whichcontain a substantial content of synthetic cis-1,4-polyisopreneelastomer while not seriously affecting the rubber composition'svulcanized properties.

While various embodiments are disclosed herein for practicing theinvention, it will be apparent to those skilled in this art that variouschanges and modifications may be made therein without departing from thespirit or scope of the invention.

What is claimed is:
 1. A rubber composition comprised ofcis-1,4-polyisoprene elastomer which contains from about 0.1 to about 20phr of a polymeric di-maleamic acid and where said cis-1,4-polyisopreneis selected from natural, cis 1,4-polyisoprene rubber, syntheticcis-1,4-polyisoprene rubber or their mixtures; wherein said polymericdi-maleamic acid is characterized as a polyoxypropylene polymer withmaleamic acid termination on each end of the polymer and having amolecular weight in a range of about 210 to 10,200.
 2. The rubbercomposition of claim 1 wherein said cis-1,4-polyisoprene elastomer issynthetic cis-1,4-polyisoprene.
 3. The rubber composition of claim 1wherein said cis-1,4-polyisoprene elastomer is a blend of natural andsynthetic cis-1,4-polyisoprene.
 4. The rubber composition of claim 1which comprises, based upon 100 parts by weight elastomers (phr), (A)100 phr of unvulcanized conjugated diene-based elastomers comprised of(1) about 10 to about 90 phr of cis-1,4-polyisoprene elastomer, whereinsaid cis-1,4-polyisoprene is selected from natural, cis-1,4-polyisoprenerubber, synthetic cis-1,4-polyisoprene rubber or their mixtures, and,correspondingly (2) about 90 to about 10 phr of at least one additionalconjugated diene-based elastomer, and (B) about 0.1 to about 20 phr ofpolymeric di-maleamic acid.
 5. The rubber composition of claim 4 whereinsaid cis-1,4-polyisoprene is comprised of syntheticcis-1,4-polyisoprene.
 6. The rubber composition of claim 4 wherein saidcis-1,4-polyisoprene elastomer is a blend of natural and syntheticcis-1,4-polyisoprene.
 7. The rubber composition of claim 4 wherein saidadditional diene-based elastomer is selected from homopolymers andcopolymers of isoprene, 1,3-butadiene or their mixtures (other than saidnatural and synthetic cis-1,4-polyisoprene elastomers), copolymers of atleast one diene selected from isoprene and 1,3-butadiene with styrene ortheir mixtures; wherein said polymeric di-maleamic acid is characterizedas a polyoxypropylene polymer with maleamic acid termination on each endof the polymer and having a molecular weight in a range of about 210 to10,200.
 8. The rubber composition of claim 4 wherein said additionaldiene based elastomer is selected from styrene/butadiene copolymerrubbers (aqueous emulsion polymerization derived and organic solventsolution polymerization derived elastomers), isoprene/butadienecopolymer rubbers, styrene/isoprene copolymer rubbers,styrene/isoprene/butadiene terpolymer rubbers, cis-1,4-polybutadienerubber, high vinyl polybutadiene rubber with a vinyl 1,2- content in arange of about 30 to about 90 percent, emulsion polymerization preparedbutadiene/acrylonitrile copolymers, 3,4-polyisoprene rubber or theirmixtures.
 9. The rubber composition of claim 4 which contains at leasttwo of said additional conjugated diene-based elastomers.
 10. The rubbercomposition of claim 1 which contains particulate reinforcement for saidrubber composition wherein said reinforcement is comprised of at leastone of rubber reinforcing carbon black and amorphous silica in an amountin a range of about 35 to about 100 phr thereof.
 11. The rubbercomposition of claim 4 which contains particulate reinforcement for saidrubber composition wherein said reinforcement is comprised of at leastone of rubber reinforcing carbon black and amorphous silica in an amountin a range of about 35 to about 100 phr thereof.
 12. An article ofmanufacture which contains at least one component which is comprised ofthe rubber composition of claim
 1. 13. An article of manufacture whichcontains at least one component which is comprised of the rubbercomposition of claim
 4. 14. A tire which contains at least one componentwhich is comprised of the rubber composition of claim
 1. 15. A tirewhich contains at least one component which is comprised of the rubbercomposition of claim
 4. 16. The tire of claim 14 where said component isselected from at least one of a tread, sidewall, carcass ply and apex.17. The tire of claim 15 where said component is selected from at leastone of a tread, sidewall, carcass ply and apex.
 18. A tire having atread comprised of the rubber composition of claim
 1. 19. A tire havinga tread comprised of the rubber composition of claim 4.